Abstract

It is widely recognised that second harmonic generation is forbidden in macroscopically isotropic media - indeed it is for this reason that surface SHG has become a powerful probe of interfaces. It is initially surprising then that there are a number of reports of a moderately strong SHG signal from dilute solutions of the purple membrane. We will report the results of our theoretical and experimental studies of this signal. One key result of the theory, which is confirmed by experiment, is that the SHG signal is retained under the condition of a circularly polarised fundamental. In an isotropic system such a signal would normally be regarded as additionally forbidden on the basis of angular momentum conservation, hence the doubly forbidden SHG signal of the title.

© 1996 IEEE